Many articles today are made using extrusion processes. Extrusion is generally a continuous process whereby a material, such as a thermoplastic, is conveyed in a melted form through a die that has a desired cross section. The material exits the die and quickly cools into its final, solid form. Thus, extrusion is an effective continuous process for making products such as tubes, films, piping, weather-stripping, window frames, and other articles having a constant cross-section. For such articles, the extrusion process is efficient, convenient, and can run continuously to produce high volumes of product. Extrusion differs from discontinuous batch processes such as injection molding, that make single, separate articles, and can produce articles having complex shapes without much effort.
Articles that vary in their material composition or in the cross-sectional shape may also be produced via extrusion, however. For example, the cross-sectional diameter of a tube may be varied by varying the air pressure introduced into the interior of the tube as it leaves the extruder die. Also, the materials being fed to the extruder may be changed in process or in situ if different parts of an article require different material properties.
The extrusion of discontinuous articles is much more complicated than continuous extrusion of constant cross-section and constant material articles. Articles formed by continuous extrusion may generally be produced by establishing the production parameters through trial-and-error when an extrusion line is first started. The extruder may then be operated continuously with little attention from the operator, except for infrequent and minor variations and corrections. The material that must be discarded during the initial set-up and calibration is generally very small in relation to the total product produced. On the other hand, the conditions in a discontinuous extrusion system are, of necessity, always changing. As a result, various transient conditions can encroach on the production process. These conditions must be accounted for before the article of interest exits from the extruder system. And because the percentage of wasted material is best kept to a minimum, the slow, trial-and-error calibration method is generally not appropriate for discontinuous extrusion operations. Moreover, it can be difficult to produce quick transitions in materials or in the extrusion geometry. These difficulties in producing a satisfactory discontinuous article by extrusion are compounded when the article being produced has extremely tight tolerances (whether for geometry, materials, or performance). The problems are further compounded over time as a system operates, because the various system parameters may each drift away from their appropriate set points.
As one example, angioplasty balloon catheters may be produced in part or in whole by extrusion. Such catheters are generally comprised of a small, flexible, but strong, hollow plastic shaft, with an expandable balloon mounted at its distal end, along with a tapered, flexible tip. The catheter must generally be capable of being threaded through a vessel or artery so that the balloon may be placed at a site in the patient, for example, where there has been arterial stenosis. To provide the required combination of flexibility and strength, it may be necessary to form different parts of the catheter out of different materials that have different degrees of compliance, or softness, within a single catheter assembly. It may also be preferable to form a single piece possessing gradual, consistent, property transitions instead of producing multiple pieces and then joining them together, such as by sonic welding or with adhesive bonding. Improved profile dimensions may also be achieved by forming the balloon with its tapered tip in the same process as the balloon and the shaft. Each of these steps requires discontinuities in the material leaving the extruder, whether in terms of material or geometry, and creates difficulties in controlling the extruder output.
Systems that simply monitor processing variability and react to maintain a desired part dimension are generally inadequate to produce articles having discontinuities and very tight tolerances, where numerous independent process parameters can affect numerous attributes of the article. As a result, there is a need for a system and method that provide accurate control over a discontinuous extrusion process.